Portable but advanced things:
Time types: SRFI 19 and/or TimeAdvancedCowan with TimePeriodsCowan
Formatting: http://synthcode.com/scheme/fmt/ FmtShinn or SRFI 28 (basic) or SRFI 48 (intermediate), possibly with SRFI 29 or GettextCowan
Binary I/O: BinaryIoCowan
Character conversion: CharacterConversionCowan
Parallel promises: ParallelPromisesCowan
Pathname objects: PathnamesPython
URI objects: UrisGauche
Unicode character database: UcdCowan, AdvancedUcdCowan
Environment: MiscEnvironmentSchudy